1. Field of the Invention
The present invention relates to a plating apparatus for plating outer leads of resin-encapsulated semiconductor devices and, more particularly, to a jet plating apparatus for spraying an electrolytic solution onto outer leads.
2. Description of the Related Art
Generally, in the manufacture of resin-encapsulated semiconductor devices, exterior plating (plating for outer leads) is performed for molded (resin-encapsulated) semiconductor devices to protect the surfaces of outer leads and facilitate soldering in mounting.
A conventional exterior plating step for resin-encapsulated semiconductor devices adopts a method of dipping resin-encapsulated semiconductor devices in the form of a multiple lead frame hung on a rack into an electrolytic solution, or a method of performing plating for resin-encapsulated semiconductor devices in the form of either a multiple lead frame or parts separated therefrom by using a barrel.
Since the above methods require a long time for performing plating, i.e., a long plating time, it is necessary to increase the productivity by increasing the number of semiconductor devices that can be processed at one time (e.g., by performing the processing for several tens frames simultaneously).
Increasing the number of semiconductor devices that can be processed at one time, however, leads to degradation in quality of plating (e.g., variations in plated film thickness). In addition, since the size of a plating apparatus is increased, it becomes difficult to incorporate the exterior plating step into an assembly line, i.e., to realize an in-line system.
There is another known method in which silver plating (or gold plating) is performed on one surface of an island portion of a lead frame and one surface of a peripheral inner lead before resin encapsulation. In this method, as shown in FIG. 1, a portion except one surface of an island portion 51 of a lead frame and one surface of a peripheral inner lead 52 is masked with a mask portion 50, and a silver electrolytic solution (or a gold electrolytic solution) 54 is sprayed from a nozzle 53 of a jet plating apparatus toward the island portion 51, thereby performing plating. In this case, the silver electrolytic solution 54 sprayed from the nozzle 53 is drained through a drainage 55 formed around the nozzle 53.
Because the method requires only a short time for performing plating, it may be applicable to the exterior plating step for resin-encapsulated semiconductor devices.
If, however, the above conventional jet plating apparatus for performing plating on a flat surface, such as the surface of a lead frame, is used to perform plating for objects to be plated having uneven surfaces, such as those processed in the exterior plating step for resin-encapsulated semiconductor devices, no uniform plated film thickness can be obtained throughout the outer leads.
As described above, if the conventional jet plating apparatus is directly used in solder plating for outer leads of resin-encapsulated semiconductor devices, no satisfactorily high quality of plating can be obtained.